A ceramic honeycomb structure is produced, for instance, by extruding a cordierite-based ceramic, moldable material through a die for molding a ceramic honeycomb structure (hereinafter referred to as “molding die” below) in a direction from moldable-material-supplying holes to molding grooves, to form a ceramic honeycomb molding, and drying and sintering it. As shown in FIGS. 4(a) and 4(b), a molding die 31 comprises molding grooves 12 arranged in a lattice pattern, and moldable-material-supplying holes 13 communicating with the molding grooves.
As shown in FIGS. 5(a), 5(b) and 5(c), supply holes 13 are arranged in the molding die 31 such that they overlap the intersecting portions 33 of molding grooves. A ceramic, moldable material introduced into the molding die 31 through the supply holes 13 is formed to a honeycomb shape by the molding grooves 12. The supply holes 13 are communicating with every intersecting portions 33 of the molding grooves 12 arranged in a lattice pattern, or with every other intersecting portions 33 in a checkerboard pattern [see in FIGS. 5(a), 5(b) and 5(c)].
In order that the ceramic honeycomb structures for cleaning exhaust gases from automobiles have larger opening areas of exhaust gas paths, and that their temperatures are more quickly elevated to activation temperatures when carrying catalysts, their cell walls are as thin as 0.05-0.5 mm. Accordingly, molding grooves 12 in the molding die 31 have smaller width 12w [see FIG. 5(c)]. On the other hand, to prevent the deformation and bending of moldings, the depth 12d of the molding grooves 12 should be 10 times or more the width 12w. 
As the molding grooves 12 have smaller width 12w and larger depth 12d, a moldable material (shown by the arrows of thick dotted lines) supplied from the holes 13 receives larger resistance while passing through the molding grooves 12. Accordingly, the molding die is bent (exaggerated by phantom lines), and large stress is applied to overlapping portions 14b of the moldable-material-supplying holes 13 and the molding grooves 12.
JP 2006-142579 A discloses a molding die comprising pluralities of cell blocks for defining molding grooves, their pitches expanding stepwise from a center portion to a peripheral portion. JP 2006-142579 A describes that the use of this molding die suppresses cell pitch unevenness between the center portion and the peripheral portion, which is caused by deformation due to uneven heating during drying, providing ceramic honeycomb structures with uniform cell pitches.
JP 2006-88556 A discloses a die comprising molding grooves arranged in a lattice pattern, and moldable-material-supplying holes staggeringly arranged in intersecting portions of the molding grooves for communication therewith for molding a ceramic honeycomb structure, which meets the conditions of A/L=1-5, and A/D=0.05-0.3, wherein in FIG. 5(c), A represents the shortest distance between a side surface 13a of a moldable-material-supplying hole 13 communicating with one groove 12 and a side surface 14a (side surface of cell block 14) of another molding groove 12 adjacent to the above molding groove 12, L represents the longitudinal length of an overlapping portion 14b of the molding groove 12 and the moldable-material-supplying hole 13, and D represents length obtained by subtracting L from the depth of the molding groove. JP 2006-88556 A describes that this molding die has such high strength that it is not broken during die machining and the extrusion of moldings, providing ceramic honeycomb structures with improved sintering strength.
However, the molding dies described in JP 2006-142579 A and JP 2006-88556 A cannot sufficiently reduce stress applied to the moldable-material-supplying holes 13 and their overlapping portions 14b with the molding grooves 12 during extrusion. Particularly when dies having narrower molding grooves are used to mold ceramic honeycomb structures having thinner cell walls, stress is likely concentrated in the overlapping portions 14b, resulting in cracking between adjacent moldable-material-supplying holes 13, 13. Accordingly, dies suitable for molding ceramic honeycomb structures having thin cell walls are desired.